If you can find a vinyl tarp for cheap, that's your best bet. If you see a big one in a store or gas station, head in and ask what they're doing with it once the ad campaign they're using it for is done. If you luck out, they might offer it to you. If you can't find one for free, check whatever online classified listings you have for your area. I had a hard time finding new ones for sale at a store, but I remember finding them really expensive.

Otherwise, skraut had a thread recently (link) about his shooting area. He said he bought some nets from Amazon (link to seller). I ended up ordering nets from them too, they're supposed to arrive in a few days though so I can't really comment on quality just yet.

Judging by your flag, you'll need to pay international shipping though. I got wrecked on that, it was like $35 shipping for $~40 worth of items.

I also know beavboyz says he used to use old blankets, which would probably be really cheap.